This invention relates to exhaust gas sampling and, specifically, to a sampling hose assembly construction that reduces or eliminates sample gas bias due to permeation of gaseous species through the wall of the sampling hose.
Measurements of gaseous pollutants in exhaust stacks of, for example, power generating turbines, are often performed by extraction of exhaust gas through long sampling lines. To avoid condensation of moisture contained in the sample gas and losses of minor exhaust species, these sampling lines are typically heated to high temperatures, ranging from 250 to 350 degrees Fahrenheit.
It has been long known in the stack testing industry, however, that heated lines with fluorocarbon (usually Teflon®) cores often exhibit positive carbon monoxide bias when heated to temperatures in excess of 250° F. In other words, when sampling a known gas, the CO concentration measured at the outlet of the heated line is higher than the CO concentration at the inlet of the heated line. For example, when sampling high purity nitrogen with zero expected CO concentration, the indicated CO concentration can range from 0 to 10 parts per million (ppm), depending on line characteristics (length, diameter, temperature, type of Teflon® material used, wall thickness, etc.).
With increasing demands for accuracy of pollutant measurements, these biases introduced by heated lines quickly become intolerable, especially for measurements of low-level pollutant sources such as gas turbines. The search continues to eliminate or at least significantly reduce heated line bias. To this end, various designs of heated lines and different materials of construction have been proposed. For example, a typical design for a heated sampling line assembly includes a single-wall sampling core (often made of Teflon®), embedded or wrapped in a heating element and surrounded by one or more layers of insulating material such as fiberglass reinforced by organic binders, e.g., phenolformaldehyde resins. One disadvantage of this traditional design is a potential for exhibiting the above-mentioned “positive bias” by the chemical species diffusing from outside of the sampling line to the inside of the sampling line and into the stream being sampled. In fact, some experimental studies suggest that the source of positive CO bias is carbon monoxide emitted by heat insulating materials at high temperature. This emitted CO diffuses from the outside of the Teflon® core to the inside of the core, increasing the concentration of CO in the sampled stream. The process is facilitated by significant increase of gas diffusivity through Teflon® material at elevated temperatures. The same process of gas diffusion through the walls of a Teflon® sampling core can lead to positively biased concentrations of other important pollutants, such as formaldehyde (albeit at lower concentrations). These situations occur when concentration of the chemical species is higher outside of the sampling core than inside the sampled stream.
Another disadvantage of these traditional sampling line assemblies is the possibility of species diffusion from the stream being sampled and accumulation on the outside of the sampling core, when concentration of the chemical species is higher in the sampled stream than outside of the sampling core. This accumulated chemical species can subsequently be released back into the sampling stream when the concentration in the sampled stream becomes lower than concentration outside of the sample core, i.e., when the chemical species concentration gradient is reversed.
Another design for an improved heated hose bundle includes using a double-wall Teflon® sampling line or tube. This approach essentially includes passing a first Teflon® line of a smaller outside diameter (e.g., ⅜″ OD) through a second and larger (e.g., ½″ OD) Teflon® line, embedded or wrapped in a heating element and surrounded by insulating materials. Aside from the intended effect of enabling a change of the inner Teflon® core if it becomes contaminated by the exhaust components (such as particulate matter or salt deposits), this design also somewhat reduces the CO bias due to the double Teflon® wall that CO has to permeate before entering the sample stream, but is subject to the diffusion problem mentioned above.
Another possible solution to the problem of positive bias due to diffusion of chemical species from the outside of the sample line into the sample stream includes use of substantially impermeable material such as stainless steel. However, chemical reactivity of stainless steel makes it unsuitable for measurements of certain chemical compounds. Alternative metals or alloys such as Inconel® or Hastealloy® usually have prohibitively high costs, while still not completely eliminating the problem of surface reactivity. Stainless steel tubing can be suitable for particular permanent sampling installations; however, to maintain line flexibility for mobile stack testing applications, the line must be constructed from corrugated stainless steel tubing. If corrugated steel tubing is used for construction of the sample core itself, it introduces significant increase of the inner surface area in contact with the sample stream, which might be detrimental for some trace gas species due to adsorption and/or heterogeneous reaction with the metal surface. Furthermore, a corrugated wall will result in a large number of pockets of essentially stagnant gas in the corrugation folds, significantly increasing sampling system response time. Increase of response time may be critical in applications such as measurements at transient combustion conditions.
These problems can be overcome by using the combination of an outer shell made of corrugated stainless steel and sample core made of Teflon®. Indeed, such a combination would virtually eliminate the positive bias by completely preventing gas diffusion from outside the metal core into the sample stream. However, the sampling hose or tube of this construction would be costly and heavy, making it less suitable for mobile stack testing applications, where cost and weight are two major factors affecting choice of the heated sample hose. Furthermore, due to significant space volume and high surface area between the inner Teflon® and outer corrugated stainless steel cores, such construction will be especially prone to accumulation of permeated gas species between the two cores and the possibility of release of these accumulated species back into the sampled stream when the concentration gradient is reversed, such as during transient or multi-point stack testing.
A further disadvantage of prior heated sampling hose designs is a possible trapping and subsequent release of gases when the sampling lines are applied intermittently for tests at sources with high pollutant concentrations and at sources with low pollutant concentrations (for example, CO and aldehydes). During sampling at sources with high pollutant concentrations, CO and aldehydes would diffuse out of the sample stream through the Teflon® wall and accumulate in the heated line assembly outside the Teflon® sampling hose. These gas species would be trapped there when the heated sampling hose is cooled down at the end of the test and Teflon® gas permeability drops. If later this heated sampling hose is used at a relatively “clean” source with low CO and aldehyde concentrations, the compounds previously trapped would be released when the sampling hose is re-heated, and would tend to diffuse from the outside of the sampling hose into the sample stream due to concentration gradient. This would result in positive measurement bias at the low-level source due to previous exposure to the high-level source. Similar effects could occur when measurements are performed during transient regimes of a combustion source, biasing high-level measurements low and, conversely, biasing low-level measurements high.